Measurement methods for longitudinal surface strain in trees: A review

Abstract

There are three common methods for measuring surface strain as a consequence of stress release in trees: (a) the Nicholson technique, (b) the CIRAD-Forêt method, and (c) by strain gauges. This article describes how each method works, the extent of its uses, why measurements are not directly comparable between methods, the source of known and potential errors, and the advantages and disadvantages of each method based on our own experience. Each method has unique advantages and disadvantages. The Nicholson method is able to determine the average longitudinal strain within a volume of wood and the ‘simplified’ version of the procedure is suitable for a large number of field measurements without causing severe damage to the cambium. The CIRAD-Forêt method does not measure strain but it gives a good estimate of the longitudinal surface strain and suits routine field measurements. The strain gauge method measures the surface strain (in longitudinal or tangential directions) directly and more accurately than the other two methods, and particularly suits fundamental research. The major disadvantage of the Nicholson and CIRAD-Forêt methods is the amount of cambium injury caused by debarking, which limits their use on small trees. Also, the Nicholson ‘primary’ procedure is error prone. The major disadvantage of the strain gauge method is the ongoing costs as strain gauges are not cheap and may not be reusable. This paper also gives a brief account of other, less widely used methods.